1. Field of the Invention
The present invention relates to the use of oximes, especially methylethylketoxime, as thermal oxidative free radical decomposition prevention additives for gels used in oil field drilling fluids, fluids used as proppant carriers and fluids used during well completion and workover.
2. Description of the Prior Art
Sols formed when a soluble dispersible gum is mixed with water find use in a wide variety of industrial applications. In a significant number of these applications it is necessary that the sol be exposed to elevated temperatures for extended periods of time.
In oil field applications the sols may become hot in use, e.g. the use of brine sols as agents to control the fluid loss in gas or oil well drilling operations or as proppant carriers and as fluid loss control agents in well completion and workover.
Unfortunately when sols of water soluble gums are exposed to elevated temperatures for any extended length of time, they lose their viscosity in part or in whole and therefore become less effective or completely ineffective. Dissolved oxygen is the major cause of an oxidative free radical polymer breakdown leading to a deterioration of the fracturing gel at elevated temperatures. Therefore, to prevent a premature viscosity degradation, oxidation inhibitors or free radical scavengers are a necessary component of fracturing gels being used in hot wells.
The normal concentration of diluted oxygen in air saturated fresh water at 20.degree. C. is about 35 g/1000 gallons which is approximately the equivalent of 1.5 lbs/1000 gal. of a typical oxidizing gel breaker. Even more oxygen is entrained in the gel and subsequently dissolved at high pressure when dry proppant is introduced into a blender and mixed with a gel. This diatomic oxygen is not very reactive at ambient pressure and temperature. However, its reactivity increases exponentially with temperature and pressure increase and becomes significant at high temperature or in deep wells.
Corrosion prevention in steam boilers makes a useful model for high temperature and high pressure fluids being pumped in oil field conditions. The strategy to control oxidation in both environments is to remove all of the molecular oxygen. The chemicals which are used to control oxygen are referred to as "oxygen scavengers". In boilers, the norm is to preheat water and to deaerate mechanically then add oxygen scavengers. In oil fields, oxygen scavengers are normally used without prior mechanical deaeration.
When first introduced, oxygen scavengers were seen as agents which "remove" dissolved oxygen. However, the "removal" of dissolved oxygen from water is actually a chemical reduction of zerovalent molecular oxygen to compounds in which this element appears in the lower -2 oxidation state. The reduced oxygen combines with an acceptor atom, molecule or ion to form an oxygen-containing compound. Hence, all oxygen scavengers are reducing agents, although not all reducing agents are necessarily oxygen scavengers.
To be suitable as oxygen scavenger, a reducing agent must satisfy the thermodynamic requirement of having an exothermic heat of reaction with O.sub.2, a condition satisfied by most reducing agents, and the kinetic requirement of a reasonable reactivity at lower temperatures, a condition not satisfied by many.
A variety of new oxygen scavengers were patented in recent years for boiler applications and several are now in commercial use. These include hydroxylamine (H.sub.2 NOH) in the form of its salts and alkyl derivatives (U.S. Pat. No. 4,067,690), Hydroquinone (U.S. Pat. Nos. 4,278,635 and 4,282,111) and hydroquinone formulated with amines (U.S. Pat. No. 4,279,767), carbohydrazide (H.sub.2 N-NH-CO-NH-NH.sub.2) as a substitute for toxic hydrazine (U.S. Pat. No. 4,269,717), ammonium erythroborate (U.S. Pat. No. 4,419,327), and methylethylketoxime (2-butanoneoxime) (U.S. Pat. No. 4,487,745).
In oil field fracturing operations, the kinetic requirements are very important. The oxygen scavenger has to remove available oxygen at low temperatures before it can damage vulnerable polysaccharides at higher temperatures. Oil field applicable oxygen scavengers must have a limited lifetime. They should be consumed in the course of a treatment so that they do not interfere with the after treatment gel breaking process. Most catalytic type, preventive antioxidants have a long life time and consequently do not qualify for oil field application.
Chemical incompatibility between reducing agents and crosslinkers creates another product choice limitation. Since all oxygen scavengers are reducing compounds, they are electron donors. Electron donors are Lewis bases capable of chelating metals which makes them unsuitable for application in metal crosslinked gels. Based on the above limitations, most common antioxidants have been rejected from oil field applications.
The most common gel stabilizer currently used in oil fields, sodium thiosulfate, can be oxidized to two products by two reactions. The first reaction, oxidation to tetrathionate, is fastest, least efficient and is usually the dominant reaction. EQU 2Na.sub.2 S.sub.2 O.sub.3 +1/2O.sub.2 +H.sub.2 O .fwdarw.Na.sub.2 S.sub.4 O.sub.6+ 2NaOH
The second reaction, the oxidation to sulfate ion, requires high temperatures to occur and is usually not significant. EQU Na.sub.2 S.sub.2 O.sub.3 +2O.sub.2 .fwdarw.Na.sub.2 SO.sub.4 +H.sub.2 SO.sub.4
Twenty parts of sodium thiosulfate per part of oxygen are required for oxygen scavenging according to stoichiometry in the first reaction.
The oxidation reaction of methylethylketoxime is as follows: EQU 2MeEtC.dbd.NOH+O.sub.2 .fwdarw.2MeEtC.dbd.O+N.sub.2 O+H.sub.2 O
Only 5.5 parts of methylethylketoxime per part of oxygen are necessary and therefore methylethylketoxime is almost 4 times more efficient than sodium thiosulfate for oxygen scavenging.